Tuesday, December 14, 2010

Should we cut Darwin out of parts of the human skin color story?

My freshmen students and I have just spent a semester reading through Nina Jablonski’s book Skin: A Natural History in which she lays out a hypothesis for the evolution of human skin color variation based on natural selection, a.k.a. Darwinian evolution.

[I think it's a fantastic book for introducing anthropology to freshmen (written by a dear friend who I happen to also greatly admire), so I built a class around it.]

The Darwinian explanation* for human skin color goes like this...

Where there is intense UV radiation (the tropics) people adapted to its destructive powers by evolving natural sunscreen, that is, lots of melanin in their skin.

Conversely, in areas where there is relatively little UV (away from the tropics, going towards the poles), people lost pigmentation in order to maximize the sun’s stimulation of Vitamin D synthesis in the skin (something that melanin inhibits).

Your skin color is about the UV environment of your ancestors. Thus, Seal and Heidi Klum are explained.

As all adaptive scenarios need be, these phenotypes are linked to reproductive success. Highly melanized skin is the primitive condition in humans, that our common ancestor in Sub-Saharan Africa evolved post-fur loss to prevent UV radiation from destroying folate—a process that can lead to death and birth defects of offspring. Once humans began dispersing around the globe, the ones to live in low UV environs evolved poorly melanized skin in order to allow enough vitamin D to be synthesized by a mother so that her fetus could form properly and then eventually grow up to reproduce successfully too. We're specifically talking about the development of the skeleton, since vitamin D is necessary for calcium to do its thing.

That women are lighter than men around the globe supports this notion that allowing UV to penetrate the skin during pregnancy is important.

Perhaps the strongest support for this hypothesis is the stunning map of the globe that Jablonski and Chaplin put together. With some exceptions, global distribution of UV intensity is positively correlated with the amount of melanin in indigenous humans so they were able to construct pretty accurate predictions of human skin color around the world based on UV.

Seal's ancestors are from Africa, while Heidi's are from Scandanavia.

[Of course, before that, Heidi's ancestors, like Seal's and yours and mine and everyone else's, lived in Africa.]

It’s an elegant explanation for the evolution of human skin color variation, and one that has gained a lot of support. But the vitamin D aspect of the story is definitely not a hypothesis preferred by all.

And then of course, as reported here on the MT recently, even a mega-study on vitamin D can't tell us for sure what levels are required to stave off health problems, or even what those health risks are!

But it's difficult to go into the details and nuance of these issues about skin color variation and vitamin D while introducing evolution to students. For many of my students, this is the first time that they’ve learned about evolution in a scholarly setting and we perform activities to illustrate the differences between Lamarckian evolution and Darwinian selection. Of course we also discuss all the known evolutionary forces—mutation, gene flow, drift, and selection—not just selection.

Few students are able to digest all of this the first time they learn it. And regardless of the explanations for why that may be (i.e. instruction quality, lack of effort, difficulty with the concepts, too much bias and misunderstanding brought into the classroom, etc…), it takes longer than a semester to understand how natural selection works and how it does not work.

For many students, the moment that they grasp natural selection, they begin to see the world through selection-colored spectacles. Everything has a reason, much like Dr. Pangloss's philosophy in Voltaire’s Candide. And it’s not just physical features... behaviors become adaptive by default as well.

It’s fine if these ideas are understood to be hypotheses, accounting for the complexities of the genes and physiological processes that lie behind the traits, and accounting for the limitations to testing them. But all too often students blindly assume that natural selection explains EVERYTHING.

[And this leads down the slippery slope to Social Darwinism so it's not something to take lightly.]

Now, even though the adaptationist perspective is rampant, that’s not at all the pattern that emerges when students interpret and explain human skin color.

They do the exact opposite! They take natural selection and adaptation out of half of the story!

Here’s (my paraphrasing of) how many of my freshmen students answer when they are asked to explain the Darwinian folate/Vitamin D hypothesis offered by Jablonski in her book:

Natural Selection explains melanized skin in the tropics because it acts as a natural sunscreen to protect against harmful UV. However, for the non-melanized people in regions with little UV exposure… well, they don't have much melanin because “they don’t need it.”

Depending on how you interpret that (aside from the possibilities that I'm not doing my job well enough, that they're not listening in class, or that they're not doing the reading), the students are invoking genetic drift, neutral theory, or Lamarckian principles! And Darwin is totally out.

I doubt many are aware of the theoretical significance of their answers. But by erroneously explaining a Darwinian concept, they're offering us a window into their intuition.

Just so we’re all on the same page, here’s a translation of the various scenarios for the loss of a trait, like pigmentation:

If losing it is beneficial, then those who have lost it will out-survive and out-reproduce others and future generations will have more have-nots than haves. If it’s crucial to survival and reproduction, then the loss will become fixed in the population as an adaptation. - Darwinian adaptation through natural (or sexual) selection

If you don’t need it then you can lose it without issue. Reproductive success does not depend on whether or not you have the trait or not, so chance alone will determine how prevalent haves or have-nots will be in any given generation. Relaxed selection plus chance can ultimately lead to the elimination of alleles all together! Furthermore, there is a constant and low mutation rate and while selection is weeding out deleterious mutations in some genes, or while it is favoring adaptive mutations in some genes, the mutations in genes for traits that do not affect evolutionary fitness can accumulate. As long as these mutations are not harmful and purged by selection, they can disrupt the gene and either damage the trait or cause complete loss. - Genetic drift and neutral theory (both with relaxed selection)

If you don’t use it, you’ll lose it, meaning that the trait can fade within a lifetime if it's neglected. That neglected trait is passed on to future generations which will continue to experience its decline if they also stop using it, and if that’s widespread throughout the population, then that trait disappears. (It’s assumed that if a trait is not used that it’s not "needed," which is why the casual wording of this scenario can be similar to #2.) - Lamarckian evolution

The first two scenarios, #1 and #2, are widely accepted as biological phenomena, so they are valid hypotheses for the loss of pigmentation in people who live far from the tropics. The third scenario is seen by the scientific community as a misconceived foil to “real” evolution, having fascinating historical interest and useful pedagogical appeal, but that is all.

Okay then, how can we interpret what I've called this "intuitive" response by my students?

First of all, tanning is certainly enabling this muddling of Darwin. That skin color changes in response to stimulation by UV (and hormones!) and is not static during life makes it complex and matches it to UV in a non-evolutionary way, a way that they're used to assuming. A way that people around the world assume to be! Some of my Kenyan friends think that my skin would look like theirs if only I stopped wearing sunscreen lotion while I visit Kenya.

And second, if students think of melanin as natural sunscreen, then it's probably easy for them to take that metaphor too far and liken it, conceptually, to sunscreen lotion.

You apply sunscreen lotion when you need it and you don't apply it when you don't need it. You need it on the Equator, yet you don't need it as much far from the Equator. This feeds back into their evolutionary story: Melanized skin evolves to be where it's needed and it evolves away where it's not needed. This is, I think, the intuitive rationale behind my students' answers. Relaxed selection, neutral theory, and genetic drift provide the backing scientific power. Plus, pigmentation loss in other animals is overwhelmingly explained this way.

But, additionally, "need" can be one way to casually express the concept of Lamarckian "striving." Are my students really Lamarckists when they say that white people are that way because their ancestors didn't need much melanin? It's hard to say.

But I also can't help but wonder, What if their confusion is not just due to their theoretical naivete? What if a totally Darwinian explanation for human skin color variation is hard to understand because it just isn't the best explanation?

Maybe my next class should be dedicated to testing and fleshing out how we could test the adaptive hypothesis for human pigmentation loss versus the alternatives.

But even if we did know the real scenario, there would still be lingering questions...

Maybe it's a horrible under appreciation of deep time and convergent evolution...Maybe it's a gross underestimation of mutation and genetic drift...

But why do so many animals in UV-limited habitats (caves and sea floor) lose pigment? Is it just one of the few visible traits that a constant accumulation of mutations can safely derail under the watchful eye of selection?

* The Darwinian explanation for skin color variation that I describe here (called "Darwinian" because it's about natural selection acting on melanin differently in different environments), is NOTthe same as Darwin's which he discusses in Descent of Man.

Excellent post...especially in terms of students' answers being somehow "intuitive." I think there is an important role here for popular science/media in terms of accurately explaining natural selection and evolutionary principles in general. We/they are not doing a very good job.

Regarding your question about loss of pigmentation in other species, is it possible that a lack of pigmentation is some kind of primitive "default" genetic state?

'Regarding your question about loss of pigmentation in other species, is it possible that a lack of pigmentation is some kind of primitive "default" genetic state?'

It's basic mathematics of molecular evolution, the synthetic theory. Adaptions that increased melanin accumulated in an environment that favored melanin. Then people migrated to an environment that no longer favored melanin, and natural selection no longer preserved melanin genes. I suppose that you can call this a default state while I suppose there was no melanin beneath early hominid hair. Of course, if we go back far enough, then second generation star dust is the default state.:)

I see nothing non-Darwinian about the loss of what's no longer favored by selection such as melanin and eyes of cave dwelling animals. Also, I know more about the synthetic theory than the writings of Darwin, but I couldn't imagine him seeing this as a non-Darwinian explanation. Does anybody know if Darwin himself predicted the loss of previously selected traits?

By the way, if there was significant advantage for the loss on melanin in colder climates, then positive natural selection helped to reduce melanin.

When I was first learning about evolution, I would always keep myself in check with Stephen Jay Gould's spandrels visualization. Is it the arch itself, or simply the shape that emerges when you put two arches together?

(Of course, Jeffrey Kurland, in his biocultural course, would later point to the medial cleft above his lips (the philtrum) and ask, "What the hell could this possibly be an adaptation to?" Same point, different pedagogy.)

And to Pam's point: the default genetic state would have to be exceedingly ancient. Pigmentation would have to be present in order for Archean-level cyanobacteria to take advantage of it in photosynthetic processes (probably). Going back earlier, to abiogenesis, well - who the hell knows? But certainly, organisms would be bombarded with solar radiation all this time, and would need some kind of defense.

On a related note, the negative effects of low melanin in high UV regions is well known. I don't have the figures on me just now, but if you're interested, look up cancer and death rates among albino people in Tanzania. An extreme example, but possibly illustrative.

In many ways the key to all of this is the need to resist the juicy temptation to spin yarns that are difficult to test much less prove, and to accept that even if there is convincing evidence for selection on a trait, like skin color, the reasons may be multiple, variable, varying, and differing both within and between populations, and over time.

Doesn't make for good TV or simple story-telling, but it may make for more realistic science, and science is supposed to be our anchor to the realistic world. There are plenty of other fantasy worlds for anyone who craves that kind of fun to indulge in.

"It is difficult to imagine conditions of life more similar than deep limestone caverns under a nearly similar climate; so that on the common view of the blind animals having been separately created for the American and European caverns, close similarity in their organisation and affinities might have been expected; but, as Schiödte and others have remarked, this is not the case, and the cave-insects of the two continents are not more closely allied than might have been anticipated from the general resemblance of the other inhabitants of North America and Europe. On my view we must suppose that American animals, having ordinary powers of vision, slowly migrated by successive generations from the outer world into the deeper and deeper recesses of the Kentucky caves, as did European animals into the caves of Europe. We have some evidence of this gradation of habit; for, as Schiödte remarks, 'animals not far remote from ordinary forms, prepare the transition from light to darkness. Next follow those that are constructed for twilight; and, last of all, those destined for total darkness.' By the time that an animal had reached, after numberless generations, the deepest recesses, disuse will on this view have more or less perfectly obliterated its eyes, and natural selection will often have effected other changes, such as an increase in the length of the antennae or palpi, as a compensation for blindness. Notwithstanding such modifications, we might expect still to see in the cave-animals of America, affinities to the other inhabitants of that continent, and in those of Europe, to the inhabitants of the European continent. And this is the case with some of the American cave-animals, as I hear from Professor Dana; and some of the European cave-insects are very closely allied to those of the surrounding country. It would be most difficult to give any rational explanation of the affinities of the blind cave-animals to the other inhabitants of the two continents on the ordinary view of their independent creation. That several of the inhabitants of the caves of the Old and New Worlds should be closely related, we might expect from the well-known relationship of most of their other productions. Far from feeling any surprise that some of the cave-animals should be very anomalous, as Agassiz has remarked in regard to the blind fish, the Amblyopsis, and as is the case with the blind Proteus with reference to the reptiles of Europe, I am only surprised that more wrecks of ancient life have not been preserved, owing to the less severe competition to which the inhabitants of these dark abodes will probably have been exposed."(The Origin of Species Chapter 5: Laws of Variation by Charles Darwin )

Remember, Jim, that Darwin's view was Lamarckian in this context (use and disuse). In modern evolutionary biology we have a more correct explanation of the 'disuse' part: the inevitable mutational variation that will disrupt function if that disruption is not rejected by selection. If there is already variation in the relevant genes, it can increase in frequency by chance (genetic drift) to reduce the level of organization of a trait.

Loss of function by these paths can be discriminated from selection, in principle, by searching for evidence of reduced variation in the relevant genes (in those instances where they're known).

So if your point was that even Darwin knew that loss of selective maintenance would lead to loss of function, that is right, but it doesn't by itself tell us whether the loss was a positive adaptation to new environments, or was the rusting of old mechanisms that no longer matter.

Thanks, Ken. I forgot that Darwin occasionally invoked Lamarckianism.:) Perhaps that's why, as I stated earlier, I'm more familiar with the synthetic theory than the writings of Darwin: I never understood evolution until I carefully examined the basics of molecular evolution. Also, I completely agree with you when you say, "So if your point was that even Darwin knew that loss of selective maintenance would lead to loss of function, that is right, but it doesn't by itself tell us whether the loss was a positive adaptation to new environments, or was the rusting of old mechanisms that no longer matter."

And to add more to the mix, although I (and others) call the natural selection explanation for skin color and all traits that evolve through NS "Darwinian," well, Darwin's own explanation for skin color variation in Descent of Man wasn't all about natural selection! :) :) :)

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